Sulfuric acid containing sulfur trioxide is useful in a number of commercial reactions. For example, it is used in alkylation of hydrocarbons, in nitration processes for dehydration, and in the preparation of methyl methacrylate. All of these processes employ sulfuric acid containing sulfur trioxide (called oleum hereinafter) and in all of these processes the oleum becomes depleted or "spent" and needs to be regenerated. Thus in the preparation of methyl methacrylate, oleum, acetone cyanohydrin (ACN) and methanol are reacted in a two-step procedure to form a mixture of methyl methacrylate, ammonium bisulfate and excess dilute sulfuric acid. The methyl methacrylate is removed, and the mixture of ammonium bisulfate and excess dilute sulfuric acid (the mixture is called spent acid) is regenerated to produce more oleum. The spent acid is pyrolyzed to form a mixture of gaseous oxides, including sulfur dioxide; the sulfur dioxide is then oxidized to sulfur trioxide which is absorbed in concentrated sulfuric acid to form oleum. The oleum thus regenerated is recycled for use in the alkylation, nitration or methacrylate preparations referred to previously.
This application pertains to improvements in the regeneration of oleum in the above processes, as opposed to the use of oleum in the primary alkylation, nitration, or methacrylate preparations referred to above. In this regeneration of oleum, considerable amounts of fuel and oxygen, added in the form of air, are fed to a furnace in which the ammonium bisulfate and dilute sulfuric acid are pyrolyzed. Inert gases (predominantly nitrogen) in the air entering the furnace are detrimental in that: (1) they add to the heat load because they must be heated to the pyrolysis temperature along with the oxygen present; (2) nitrogen forms nitrogen oxides in the hot oxidizing environment of the furnace, thereby creating nitrogen oxide pollutants that must ultimately be discharged as part of the stack gas and generating niter, a product contaminant which reduces the yield of the resulting oleum product; (3) they dilute the concentration of SO.sub.2 in the converter (where SO.sub.2 is converted to SO.sub.3), thereby limiting conversion of SO.sub.2 to the desired SO.sub.3 and increasing SO.sub.2 discharge rate to the atmosphere as a pollutant in the stack gas; (4) they limit the strength of the oleum which can directly be produced; (5) they reduce the holdup time of the reactants in the converter, for a given throughput rate, making it necessary to use larger volumes of catalyst for the desired reaction; and (6) they cause a pressure drop in the equipment used in the regeneration process.
One object of the process improvements of this invention is to increase the capacity of a spent acid regeneration facility by reducing the mass and volumetric flow in the system and increasing the concentration of SO.sub.2 in the mass which, in turn, increases the oleum yield and concentration.
Another object is to save energy by reducing the amount of inert gases in the furnace thus reducing the amount of fuel needed to heat the mass to pyrolysis temperature and by reducing the power needed to push the mass through the entire system.
Yet another object is to reduce the pollution load by reducing the volume of stack gas, i.e., gas exhausted to the atmosphere after completion of the process, and the emission rate of pollutants, SO.sub.2, and NO.sub.x per unit of 100% acid produced.
A further object is to increase the concentration of the oleum produced without having to resort to conventional dewatering and to improve the quality of the product by minimizing the amount of niter produced.